Actually shrimp have very small bioloads, you could try a few shrimp. With bettas, ghost shrimp are usually a hit or miss thing. I don't know if it was because they were small but my betta killed two of tiny shrimpy heart attacks(he never left them alone) and one directly because he ate it. I had a good experience with bamboo shrimp which are way bigger, though i don't suggest them untill your tank is at least six months old or you would have to grind up food for them since they are filter feeders and don't have claws.

Even if you decide to not get shrimp, some small snails to help keep any algea in check ( which i myself am looking into) could help a lot. Though because of your tank size i would do research into snail that Do not produce to much waste, careful of mystery snail I've read the produce a lot of it. What ever you decide keep us posted and provide lots of pics!